The present invention relates to extrusion dies for the continuous extrusion of fine structures from plasticized materials, and more particularly to honeycomb extrusion dies comprising improved discharge slot designs imparting more uniform and stable extrusion characteristics to the dies.
The manufacture of inorganic honeycomb structures from plasticized powder batches comprising inorganic powders dispersed in appropriate binders is well known. U.S. Pat. Nos. 3,790,654, 3,885,977, and 3,905,743 describe dies, processes and compositions for such manufacture, while U.S. Pat. Nos. 4,992,233 and 5,011,529 describe honeycombs of similar cellular structure extruded from batches incorporating metal powders.
The manufacture of extrusion dies for the production of ceramic honeycombs by these methods requires extremely precise machining. To supply material to the slotted honeycomb discharge section of such a die, the inlet or supply face of the die is provided with multiple apertures or feed holes through which the plasticized batch material to be extruded is forced under high pressure. The opposing or discharge face of the die is provided with a crisscrossing array of finely machined discharge slots, these slots being cut into the discharge section of the die and intersecting the feed hole array. It is these discharge slots which shape the plasticized batch supplied to the bases of the slots by the feed holes into the interconnecting wall structure of an extruded honeycomb.
The islands of material between the intersecting discharge slots, which form the actual discharge face of the die, are sometimes referred to as "pins", since they appear as free-standing metal posts extending outwardly from the die interior and are attached to the die body only at their bases. The shapes of these pins define the shapes of the honeycomb channels formed by the extruding plasticized batch.
A number of techniques have been employed to shape metal billets into honeycomb extrusion dies. For softer steels, the feedhole array can be formed by mechanical drilling and the discharge slots by sawing. However, if the die is to be formed of harder, slower wearing materials such as stainless steels, electrochemical machining and electrical discharge machining are more widely used.
In the electrochemical machining (ECM) process, also referred to as the STEM (Shaped Tube Electrolyte Machining) process, the feed hole apertures are formed through a controlled deplating (dissolution) of the electrically conductive steel workpiece. An electrolytic cell is formed wherein the drill comprises the negatively charged electrode (cathode), the workpiece comprises the positively charged electrode (anode), and the electrolyte is a flowing electrically conductive fluid.
In die slotting by electrical discharge machining (EDM) the discharge slots are formed through an electrical discharge maintained between a long, thin, traveling electrode wire and the metal die preform. Slot lengths formed by wire EDM in these dies are generally greater than about 31/2 inches, with slot depths of 0.1 inches or more and slot widths of 0.012 inches or less.
EDM is well suited for the shaping of thin slots with parallel sidewalls in these dies. The slots are typically substantially free of burrs and have a relatively smooth and consistent surface finish. Reference to U.S. Pat. Nos. 2,526,423, 4,205,213, 4,233,486, 4,403,131 and 4,527,035 may be made for further descriptions of EDM processing.
Although ECM and EDM remain important processes for the machining of large arrays of fine apertures and discharge slots in steel blanks for honeycomb extrusion dies, problems with the resulting dies still remain. These are usually manifested as an uneven flow of material as the honeycomb preform is extruded from the discharge face of the die. Uneven flow effects, such as "fast flow" (accelerated extrusion through localized regions of a die), can produce a variety of honeycomb product defects. These include "swollen" or "rippled" webs, i.e., cell walls distorted in transverse and/or longitudinal directions due to the extrusion of excess material, as well as other variations in cell dimensions, cell shapes, and cell wall thicknesses.
Surface finish irregularities within the feed hole and/or discharge slot sections of these extrusion dies are considered to be major contributors to uneven flow. Such irregularities can be caused by very slight variations in electrical voltage, current, or other discharge conditions during the machining process, these changing from aperture to aperture and slot to slot across the inlet and outlet faces of the die. These variations cannot at present be fully controlled.
Efforts to substitute alternative machining processes for the more conventional processes used for die fabrication, in order to optimize feed hole and/or slot consistency and surface finish, have not solved these problems either. For example, slotting through the use of an abrasive wheel cutting process in place of the more conventional EDM slotting process, while not subject to process disruptions from variations in electrical discharge conditions, also failed eliminate extrudate flow variations even though slot finish was smoother and slot shape variability was reduced.